My son in 3rd Grade is considering experiment on either of the two following topics1. Will objects of different masses when dropped from same height reach the ground at the same time?2. Why does a marble roll at different speed on different surfaces?

For Option 1 above - How do we ensure the dropping of the object at the same time from 2nd floor of our house. Do we measure the time to reach the floor using a video camera?For Option 2 if we release a marble on different surfaces how do we ensure that they are released with the same force?

For Option 1 to do this measurment accurately in the lab one could use a drop timer (http://www2.selu.edu/Academics/Faculty/ ... age11.html) or a photo gate timing system (http://store.pasco.com/pascostore/showd ... 4&Detail=1). But both are expensive so as a compromise you can get reasonable results with a stop watch. Just use many (+10) repetitions to average out your data. To verify how accurate your stop watch is and your ability to quickly and accurately take measeurments record the data do the math and see if you get the number you expect which is gravitational acceleration, g = a = 9.8m/sec^2. You can drop various objects at different times just make sure you record the time of drop and the time of landing for each trial.

For option 2 to ensure that the ojects are released with the same force us no force at all. Find a ramp and release the objects with no force by just letting go of them. Gravity will do the rest.

Hello!Thanks for your suggestions. He has chosen to go with the Friction project. We have taken few planks of wood and glued various surfaces to it to test the friction. The surfaces tested are wood, wax paper, sand paper, aluminum foil, rug, cardboard.1. In the trial run we found that the toy car we rolled moved slower on wood than on wax paper. Do you think that would be true? Could this be because the surface is glued to wood? How can we eliminate this error?2. Would the results be more accurate if we test it with a marble than a toy car?Thanks

It doesn't matter what anyone thinks, that is what you found. Assuming you did at least three trials for each surface and the variance (difference from one trial to the rest of the trials on the same surface) was such that the different trials did not overlap data from trials on the other surfaces, you had a conclusive result.

So what does your result mean? That is a more difficult question. The frictional forces on wheels have at least two components, one is at the axel and the the other is at the rolling surfaces.

If the frictional force component at the axel is larger than at the rolling surface, then the wheel will be partially sliding and not fully rolling. It would be interesting to test on the wood and wax paper surfaces with locked wheels and see what the test results are if you really want to figure out which of the two frictional components had what effect. You can "lock" the wheels by taping the top portion of the side of all wheels to the toy car body so they won't rotate. You may have to increase the incline angle as the sliding frictional force may be larger than the portion of the gravitational force associated with forward movement down the incline.

anuami wrote:Would the results be more accurate if we test it with a marble than a toy car?

Yes/No/Maybe. Definitely DIFFERENT because you would be testing a different set of material pairs (assuming your tires are a different material than the marble you plan to use) and the marble doesn't have an axel and associated frictional component. Even without an axel frictional component, the marble could easily be partially sliding on some surfaces. For really hard surfaces (ones where the weigh of the marble does not cause enough pressure to deform the surface it is resting/rolling on), the marble must be partially sliding to experience any significant friction. For really soft surfaces that easily deform, the "friction" you calculate will be dominated by the energy utilized to deform the surface and the sliding friction will be insignificant.

Engineers struggle with trying to apply test results from on test case to analyzing other cases and the question is always how well does the test case that produced the data match the conditions being analyzed and what differences might be expected?